Demisters: when to use?

[Bigsfish]

I have demisters on this GB and always thought of them as a fancy blowers or exhaust for engine rooms to remove heat and any fumes. So you can imagine my surprise when a captain asked me if I used them while running to make he engine more efficient? I'm all for "more efficient " ( yeah I'm cheap) and I told him I never used them with running.

So my question to all you brains on the forum is should I be running them while operating the engines? Will they save me money while running and does the RPM make a difference?

Please educate me.

 

[gaston]

Windshield demisters ??? or engine room exhaust blowers ???

 

[gaston]

What engines? turbo or old school?

 

[gaston]

Have you calculated the difference between ambient air temp and ER temp ?

 

[gaston]

What size motor on the blowers ? are they adjustable ?

 

[RT Firefly]

Heyyyy...What about demisses? You forget about her and you sleep in de back room....

 

[dhays]

I'm a bit confused by the term "demister". It appears to be something to remove salt water vapor from the engine room?

An ER blower could reduce ER temperatures which would make your engine more efficient. Power from the engine decreases as intake temperatures rise. Lots of variables though such as ambient temperatures, whether you have a turbocharger with an aftercooler or not, and what the water temperature is in that aftercooler.

Maybe you could tell us what your boat has. Is it just a blower? Is it sucking air out of the ER or blowing are into the ER?

 

[Bigsfish]

Gaston

Engine rooom. Turbo 6.7 Cummins. Either on or off, not variable. Never calculated the temp difference but these engines run cool ( below 180).

 

[Bigsfish]

Senor RT

I swear she told me she was 18!

 

[dhays]

Gaston

Engine rooom. Turbo 6.7 Cummins. Either on or off, not variable. Never calculated the temp difference but these engines run cool ( below 180).

The Cummins suck a lot of air. They alone will create a lot of air exchange in your ER. If the air temps in the ER are significantly higher than ambient, then increasing air exchange couldn't hurt. However, if the blowers are exhausting the ER, then make sure your ER vents are sufficient to provide enough flow. With the engines running at normal cruise RPM and the blower on, see if there is any negative pressure as you open the ER door.

 

[BandB]

Here's an excellent article on demisters.

Ventilating Your Boat's Engine Room

 

[gaston]

I have 2 very big ER exhaust blowers and a turbo 6.7 Cummins. Id say the blowers are there to just make the ER a little more comfortable on a stinking hot day maybe reduce the ER temp by 5deg c ??? which is nothing .
The draw on the alternator on the Cummins to drive the 2 blowers would be greater than the HP gain from the cold air in the ER otherwise you may have found the answer to perpetual motion .
I do wonder what HP gain could be made buy fresh air intake Id guess that calculation has already been done and the difference isn't worth the effort

 

[BandB]

I have 2 very big ER exhaust blowers and a turbo 6.7 Cummins. Id say the blowers are there to just make the ER a little more comfortable on a stinking hot day maybe reduce the ER temp by 5deg c ??? which is nothing .
The draw on the alternator on the Cummins to drive the 2 blowers would be greater than the HP gain from the cold air in the ER otherwise you may have found the answer to perpetual motion .
I do wonder what HP gain could be made buy fresh air intake Id guess that calculation has already been done and the difference isn't worth the effort

The blowers bring in air which is good except that air may be moist and have salt in it. That's the reason for demisters.

 

[what_barnacles]

Here's an excellent article on demisters.

Ventilating Your Boat's Engine Room

Maybe you were thinking of his "future" column?

In a future column, I’ll explain how demisters work.

 

[gaston]

Blowers and demisters are 2 very different animals

 

[Bigsfish]

BandB

This boat has those Delta T, and they are blowers. I have never heard of running with the blowers on before so this surprised me. I don't feel the engine room is very hot while running.

 

[BandB]

BandB

This boat has those Delta T, and they are blowers. I have never heard of running with the blowers on before so this surprised me. I don't feel the engine room is very hot while running.

Delta T are very good systems.

 

[Comodave]

Delta T are great. Are they setup to intake or exhaust? Also is the ducting high or low? If they are exhaust, they may be to evacuate heat after running. If they are intake they may needed to supply combustion air for the engines. They are not demisters. Demisters are a baffle system that is convoluted so that moisture in the intake air will stick to the baffles and then drain overboard instead of getting into the ER.

 

[BandB]

Delta T are great. Are they setup to intake or exhaust? Also is the ducting high or low? If they are exhaust, they may be to evacuate heat after running. If they are intake they may needed to supply combustion air for the engines. They are not demisters. Demisters are a baffle system that is convoluted so that moisture in the intake air will stick to the baffles and then drain overboard instead of getting into the ER.

Delta T makes demisters too. They make most any type ventilation or fan system.

 

[Capt.Bill11]

BandB

This boat has those Delta T, and they are blowers. I have never heard of running with the blowers on before so this surprised me. I don't feel the engine room is very hot while running.

First, do they blow in or suck out?

If the suck out you don't want to run them while underway. They will be fighting the engines for fresh air.

After you shut down, sure. To help cool the engine space. But be careful, because sometimes they can draw air from the interior of the boat and suck your cool aircon air out at the same time as they are sucking out hot ER air.

If the blow into the engine space then running them underway can be a good thing. But not if they over pressurize the engine room so much that they force hot air/odors into the the boats interior.

Variable speed/reversible fans are the best choice.

 

[Comodave]

Yes, Capt Bill that was the point I was going toward with my question about in of out air flow. We need to know how the blowers are plumbed before we can make knowledgeable recommendations.

 

[dhays]

I am learning from this discussion. Thanks for the question Bigsfish. I now have a very general idea of what a demister is. Seem to me that would be more necessary for a go-fast boat with turbo charged twins than a slow single such as mine.

I have at times run the ER blower after shutting down the engine in warm weather in an attempt to cool the ER somewhat. Otherwise there is large heat source until the saloon floor.l

 

[FF]

"If the blow into the engine space then running them underway can be a good thing. But not if they over pressurize the engine room so much that they force hot air/odors into the the boats interior."

ANY pressure in the engine room will be forced into the rest of the of the boat , bringing the ER smells with it.

It will also bring along any leaking exhaust gasses.

With proper vents , I have never heard of a blower with the ability to raise the altitude in the ER, so reduce engine power.

 

[Ski in NC]

I have been hired to do engineering reviews on engine room ventilation, especially when someone makes a claim that there is inadequate air supply or excessive temperatures. In all these cases, it turns out that at high power settings, the incredible amount of air drawn into turbos brought engine room temps way down and differential pressure measurements showed no significant vacuum. And unless blowers are HUGE, they have little effect on either of these concerns.

At low speeds, engine room temps can go way up as the turbo suction flow rate drops way off. But in that case, the engine does not care one bit. It is getting way more oxygen than it needs.

Demisters are some sort of technique where incoming engine room air either goes through baffles or a coalescing filter to knock out the salt mist. Important stuff. And does not involve fans. If after a hard run at sea you find salt dust on your engine room machinery, your boatbuilder did not do it right!! And retrofitting proper air handling stuff can be a major PITA.

With regards to efficiency, unless engine is showing signs of exhaust smoke (unburnt fuel), it has enough air to burn efficiently. Blowers will not do anything but eat power, make noise and wear themselves out. There will be no significant change in fuel consumption.

Blowers are useful for creature comfort if you have to go in the ER after shutdown. Or if running generator with main engine off. Gen end is air cooled and that heat can build up in ER. Good to run a blower then.

 

[Bigsfish]

Well I guess I have this fancy system that I will continue to use to cool the ER before working in it. LOL. They must work because they are loud! ��

Thanks all.

 

[Capt.Bill11]

"If the blow into the engine space then running them underway can be a good thing. But not if they over pressurize the engine room so much that they force hot air/odors into the the boats interior."

ANY pressure in the engine room will be forced into the rest of the of the boat , bringing the ER smells with it.

It will also bring along any leaking exhaust gasses.

With proper vents , I have never heard of a blower with the ability to raise the altitude in the ER, so reduce engine power.

Yes with proper sized vents it's a nonissue. But if the ER vents are under sized for the intake air needs of the engines at higher speed operations, which is not uncommon, then running exhaust blowers could further restrict the engines ability to get ambient temperature intake air.

Not all boats have little dc blowers. Some have large 120v Delta T type blowers which can move significant volumes of air.

 

[Comodave]

I had a 46' trawler with twin Detroits that would get up to about 130 degrees on a long run. The boat did not have any blowers, but would have beed nice to have one to help evacuate the heat after shutting down. If I had kept the boat, I would have put in more intake venting.

 

[ranger58sb]

Well I guess I have this fancy system that I will continue to use to cool the ER before working in it. LOL. They must work because they are loud! ��

FWIW...

Our blowers were very loud. Originally mounted directly on hard cross members supporting the saloon sole.

I fashioned some stand-offs using 1½" sanitation hose... so the hoses are mounted to the cross members, and the blowers are mounted to the hoses. Cut noise level by about 80% (subjective "measurement").

First read about that on line somewhere, maybe here...

-Chris

 

[dhays]

FWIW...

Our blowers were very loud. Originally mounted directly on hard cross members supporting the saloon sole.

I fashioned some stand-offs using 1½" sanitation hose... so the hoses are mounted to the cross members, and the blowers are mounted to the hoses. Cut noise level by about 80% (subjective "measurement").

First read about that on line somewhere, maybe here...

-Chris

Good idea. My ER fans are a bit noisy as well. The last thing we want after getting our anchor set is the noise of the fans. The genset is much quieter and we even don't like that.

 

[twistedtree]

I think Ski nailed most of it. You can have any combination of power or passive for intake or exhaust vents. Plus of course the engine(s). Ideally the ER will run at a slight negative pressure so smells, heat, and fumes get draw out of the boat via the vents rather than pushed into the interior of the boat.

Many equipment manufacturers specify an ER temp of no more 130F, and no more than 30f temp rise over ambient. 130F is also at the point of being dangerous if you need to spend any time in there.

Sizing of ducts is also very important. Just using bigger fans on an undersized duct will just consume electricity, and typically end up making the ER hotter, not cooler.

I recently wrapped up a two year project to get my ER operating temp under control and wrote a blog article about it if anyone is interested. It's on MVTanglewood.com.